In recent years, a liquid crystal display device has been widely used for providing a display of a personal computer, a word processor, a terminal display device for office automation, and a television and others thanks to its low power consumption, thin structure, and light weight. Accordingly, a display with a larger capacity and higher picture quality has been demanded.
A conventional liquid crystal display device has performed a simple-matrix driving in accordance with a voltage averaging method of the STN(Super Twisted Nematic) system. However, since a sufficient contrast ratio cannot be obtained due to the increase of scanning lines, this method is not suitable for a display with a large capacity. Therefore, an active-matrix driving has been developed to provide a switching element on each of the pixels constituting a display screen.
As the aforementioned switching element, a thin-film transistor and a two-terminal nonlinear element are used, and the liquid crystal display device with the two-terminal nonlinear element has been highly evaluated because of the simple construction and low manufacturing cost. A device having a structure of metal-insulator-metal (hereinafter, referred to as MIM) has been put into practical use. For example, a conventional MIM element is produced as follows:
Firstly, as shown in FIGS. 19 and 20, on a glass substrate 51, a tantalum thin-film, which is formed to be a signal wire 52 and a lower electrode 53, is laminated with a thickness of 3000 .ANG. in accordance with a sputtering method, etc., and is patterned into a predetermined form so as to obtain the signal wire 52 and the lower electrode 53 by a photolithography method. Thereafter, in accordance with an anodic oxidation method, an anodic oxidation is performed on the surface of the lower electrode 53 so that an insulating film 54 is formed with a thickness of 600 .ANG..
And then, in this state, titanium formed to be an upper electrode 55 is laminated on the entire area of the substrate with a thickness of 4000 .ANG. by the sputtering method, etc., and is patterned into a predetermined form by the photolithography method so as to be formed as the upper electrode 55.
Further, a transparent electrode film which is made of ITO and others is laminated on the substrate, and the substrate is subjected to patterning so as to form a pixel electrode 56. FIG. 21 illustrates an arrangement pattern of pixels composed of MIM elements and pixel electrodes 56, said MIM elements and pixel electrodes 56 being formed in accordance with the aforementioned process.
As described in FIG. 21, in the liquid crystal display device which uses the matrix driving system, a parasitic capacity appears in each pixel due to the effects of neighboring pixels and wires. Further, in addition to the case of the aforementioned liquid crystal display device which uses a two-terminal element such as an MIM element and others, the parasitic capacity appears to some extent even in the case of a liquid crystal display device using other active elements or in the case of a liquid crystal display device using the simple-matrix driving system. Especially, in the case of the liquid crystal display device which uses the two-terminal elements, the effect of the parasitic capacity becomes the greatest.
That is, in the case of the liquid crystal display device using the two-terminal element as a switching element, when an element capacity varies in accordance with a change in element dimensions and others of the two-terminal element, a threshold voltage V.sub.th varies accordingly; therefore, a lighting condition differs between pixels. Consequently, with regard to all pixels, it is significant to obtain uniform capacity ratios of a pixel capacity and the element capacity. The pixel capacity is a capacity of the pixel electrode 56, and the element capacity is a capacity of the MIM element. Here, the parasitic capacity greatly influences the capacity ratio.
The specific explanation will be given in accordance with FIG. 21. Based upon one pixel(sample pixel) among a plurality of pixels which are arranged in a matrix form, signal wires 52a and 52b, which are respectively disposed on the left and right of the sample pixel, add respectively parasitic capacities C1 and C2 to a pixel electrode 56a of the sample pixel. Further, a pixel electrode 56b of a pixel which exists above the sample pixel adds a parasitic capacity C3, and a pixel electrode 56c of a pixel which exists below the sample pixel adds a parasitic capacity C4.
The parasitic capacities C1 through C4 are added to the pixel capacity of the pixel electrode 56a, thereby having an effect on the capacity ratio of the pixel capacity and the element capacity. Namely, with regard to the respective pixels, even if element dimensions of the two-terminal element are arranged so as to be uniform in order to keep the element capacity at a certain amount, the capacity ratio of the pixel capacity and the element capacity varies in accordance with a change in the parasitic capacity.
However, in the conventional arrangement, as shown in FIG. 22, pixel electrodes 56d, which exist on the right end, do not have the signal wire 52 on its right; therefore, the parasitic capacity C2 is not added. Further, pixel electrodes 56e, which exist on the upper end or the lower end, do not have the pixel electrodes 56 above or below; thus, the parasitic capacity C3 or C4 is not added. Moreover, in the case of pixel electrodes 56f, which exist on the upper right corner or the lower right corner, parasitic capacities C2 and C3 are not added, or parasitic capacities C2 and C4 are not added.
Hence, with regard to the liquid crystal display device which uses the aforementioned MIM element, each of the parasitic capacities which are applied to the pixel electrodes 56d, 56e or 56f is smaller than the parasitic capacity which is applied to the pixel 56a, illustrated in FIG. 21. Consequently, since a lighting condition differs between pixels, it becomes impossible to provide an even lighting display.
Namely, as illustrated in FIG. 22, with regard to pixels which exist on the upper end, lower end, and on the right end(pixels indicated by slanting lines in FIG. 22), the threshold voltage V.sub.th becomes lower, resulting in an uneven display. Therefore, for example, in the case of a gradation display, this arrangement causes adverse effects.